Various devices are well known for providing a reference frequency or source. Such devices are called oscillators and typically incorporate a quartz crystal or other type of resonator and electronic compensation circuitry to stabilize the output frequency.
Various methods are known to stabilize the output frequency as the temperature of the oscillator changes. Temperature compensated crystal oscillators (TCXOs) typically employ a thermistor network to generate a correction voltage which reduces the frequency variation over temperature. The correction voltage is usually applied to a varactor diode in the crystal circuit such that the crystal frequency may be varied by a small amount. TCXO stability can approach 0.1 PPM but several problems must be addressed.
A TCXO that resides at one temperature extreme for an extended period of time may exhibit a frequency shift when returned to normal room temperature.
Usually this hysteresis or “retrace” error is temporary but a seemingly permanent offset is common. Retrace errors are usually less than about 0.1 PPM but can be much higher, especially if the mechanical tuning device (trimmer capacitor or potentiometer) is shifting. This hysteresis makes the manufacture of TCXOs with specifications approaching 0.1 PPM quite difficult. The high precision crystals found in oven oscillators exhibit less retrace but they are unsuitable for use in TCXOs because they often exhibit activity dips at temperatures below the designed oven temperature.
Further SC-cut and high overtone type crystals cannot be tuned by a sufficient amount to compensate for the frequency excursion with temperature. In addition, SC-cut crystals are very expensive.
TCXOs are preferred to oven oscillators in low power applications and where a warm-up period is not acceptable. The only warm-up time is the time required for the components to reach thermal equilibrium and the total current consumption can be very low—often determined by the output signal power requirements. Older TCXO designs employ from one to three thermistors to flatten the crystal temperature frequency curve. Newer designs employ digital logic or a microprocessor to derive a correction voltage from values or coefficients stored in memory.
Ovenized oscillators heat the temperature-sensitive portions of the oscillator which are isolated from the ambient to a uniform temperature to obtain a more stable output. Ovenized oscillators contain a heater, a temperature sensor and circuitry to control the heater. The temperature control circuitry holds the crystal and critical circuitry at a precise, constant temperature. The best controllers are proportional, i.e., providing a steady heating current which changes with the ambient temperature to hold the oven at a precise set-point, usually about 10 degrees above the highest expected ambient temperature.
Temperature-induced frequency variations can be greatly reduced by an amount approaching the thermal gain of the oven. The crystal for the oven is selected to have a “turning-point” at or near the oven temperature, further reducing the sensitivity to temperature. The combination of the high oven gain with operation near the turning point yields temperature stabilities approaching 0.0001 PPM over a temperature range that would cause the crystal to change by 10 PPM.
Highly polished, high-Q crystals which often have significant activity dips may be designed with no activity dips near the operating temperature, providing better stability and phase noise than crystals designed for wide temperature ranges. Ovenized oscillators allow the use of SC-cut crystals which offer superior characteristics but which are impractical for ordinary TCXOs because of their steep frequency drop at cooler temperatures. Unfortunately, SC-cut crystals are much more expensive to produce than AT-cut crystals typically used in TCXOs.
Oven oscillators have a higher power consumption than temperature compensated oscillators. Oven oscillators require a few minutes to warm up, and the power consumption is typically one or two watts at room temperature. SC-cut crystals stabilize as soon as they reach the operating temperature, but AT-cut crystals exhibit a significant thermal transient effect, which can take many minutes to settle. A typical AT-cut crystal will drop in frequency well below any point on the static crystal curve due to the sudden application of oven heat. In most oscillators, the frequency will exponentially drift back up to the nominal frequency.
In some designs, the oven controller overshoots significantly during initial warm-up and then cools back down to the final oven temperature. This cooling transient can kick the AT-cut crystal in the other direction and may actually result in a shorter warm-up time even though the controller takes longer to settle. Hand-tweaked designs can achieve fairly acceptable warm-up times with carefully selected overshoot, but the advent of quick-settling SC-cut crystals has made this approach obsolete.
Despite the advantages of prior art oscillators, an unmet need exists for an oscillator that has improved frequency stability over temperature and time and that can be manufactured at a reasonable cost.